Injection molding processes for performing sequential shots of different polymer materials are well known. To accomplish such processes, injection molding apparati have been developed using hotrunner systems that are designed to deliver sequential shots of polymer material both to a single cavity and to a plurality of cavities. In multi-cavity applications, shots are intended to be delivered at the same time in the same amounts and at the same rates of flow by controlling the length and configuration of the hotrunner flow channels and the temperature of various portions of the hotrunner and the injection nozzles and the mold cavity itself. However, in practice, it is very difficult to achieve such uniform delivery to multiple cavities.
When shots of plastic materials are delivered in sequence to even a single cavity, it can be difficult to obtain consistency in the precise amount of the shots from one cycle to the next. When shots are routed through multiple flow channels in a hotrunner leading to multiple cavities, it is even more difficult to maintain precise control over the pressure within any given channel or injection nozzle or mold cavity and thus the rate and/or volume of material flow to any particular one of the plurality of cavities will vary. When a single source of polymer material injection is used to effect flow through all channel paths to each mold cavity, pressure will vary between the flow paths even at points within different channels that are located the same distance (path length) from the source of injection. Performing two or more shots of material one after the other further increases the degree of difference of volume of polymer material that is delivered to different cavities in each shot. Still further, changes in the polymer material(s) over time (e.g., different batches, sources, temperatures, moisture content) can alter the flow characteristics even for a specific hotrunner/cavity path.
Prior systems describing typical sequences of injecting sequential shots of first, second and/or third shots of polymer materials into mold cavities and the apparati used to effect such multi-cavity injection are set forth in U.S. Pat. Nos. 4,550,043; 4,609,516; 4,710,118; 4,781,954; 4,950,143; 4,990,301; 4,923,723; and 5,098,274, the disclosures of all of which are incorporated herein by reference as if fully set forth herein.